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A = angle A

B = angle B

C = angle C

a = side a

b = side b

c = side c

P = perimeter

s = semi-perimeter

K = area

r = radius of inscribed circle

R = radius of circumscribed circle

Uses the law of cosines to calculate unknown angles or sides of a triangle. In order to calculate the unknown values you must know and enter 3 known values.

To calculate any angle, A, B or C, enter 3 side lengths a, b and c. This is the same calculation as Side-Side-Side (SSS) Theorem. To calculate a side, say a, enter the opposite angle A and the to other adjacent sides b and c. Using different forms of the law of cosines we can calculate all of the other unknown angles or sides. This is the same calculation as Side-Angle-Side (SAS) Theorem.

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If a, b and c are the lengths of the legs of a triangle opposite to the angles A, B and C respectively; then the law of cosines states:

## Law of Cosines solving for sides a, b, and c

## Law of Cosines solving for angles A, B, and C

Triangle perimeter, P = a + b + c

Triangle semi-perimeter, s = 0.5 * (a + b + c)

Triangle area, K = √[ s*(s-a)*(s-b)*(s-c)]

Radius of inscribed circle in the triangle, r = √[ (s-a)*(s-b)*(s-c) / s ]

Radius of circumscribed circle around triangle, R = (abc) / (4K)

Weisstein, Eric W. "Law of Cosines" From *MathWorld*-- A Wolfram Web Resource. http://mathworld.wolfram.com/LawofCosines.html

Zwillinger, Daniel (Editor-in-Chief). *CRC Standard Mathematical Tables and Formulae, 31st Edition* New York, NY: CRC Press, p. 512, 2003.

**Cite this content, page or calculator as:**

Furey, Edward "Law of Cosines Calculator" From *http://www.CalculatorSoup.com* - Online Calculator Resource.